This invention discloses the use of a direct drive linear motor that is submersed in oil and is supported on its own hydrodynamic oil bearings. All wear and metal to metal contact are eliminated. This feature is unique in the field of artificial hearts or left ventricular assist devices.
Implantable blood pumps have been worked on for several decades. These have been funded in part by the U.S. National Institutes of Health. Although much progress has been made no approach has demonstrated the high reliability needed for the actuator. Generally, the motion of a rotary electric motor is converted into the linear motion of a pusher plate to squeeze blood from rubber type ventricles. Some move on hydraulic piston which squeezes the ventricles with fluid. Some push on the ventricles directly using no hydraulics. The rotary to linear conversion mechanism such as lead screws, gear pumps and a host of other designs are prone to wear, and possess many moving parts such as the support ball bearings. These complex approaches, with many moving parts, substantially reduce the mechanical reliability attainable. Actuator approach complexity has been the major stumbling block in attaining a highly reliable, light weight prosthesis.
The proposed use of a linear direct drive like a solenoid is not new. This circumvents the rotary to linear conversion problem but no one has demonstrated a long life and efficient design. This is because most linear actuators, such as solenoids, produce large side forces and therefore require massive support bearings. Roller or ball bearings are typically used in commercial linear motors, but their use is undesireable compared to the non-wearing hydrodynamic approach of the instant invention.
Direct drive linear electric actuators have also not been used in implantable blood pumps because they have been inefficient. It is desireable to minimize the power consumed if the patient is to be truly mobile, as he is dependent on a wearable battery source. Existing linear motors are typically of the reluctance type. They are not generally efficient at the slow velocities required for ejecting blood in a direct drive. Voice coil linear motors on the other hand operate on the Lorentz force generated by a current in a coil interacting with a magnetic field. The diaphragm drive in loud speakers is an example. The loud speaker coil moves only a small amount and is attached to the diaphragm. Prior to this invention voice coil linear motors have not been used in blood pumps because they were believed to be too inefficient for the relitively long stroke and high forces required. The coil leads in loud speakers are prone to fatigue breakage. Moving coil reliability is unacceptable in a longer stroke device such as a blood pump. It is also undesireable to use conductive spring arrangements to supply the power for reliability concerns. The proposed actuator uses a moving magnet instead with a stationary coil and has no such reliability concern.
Furthermore, the proposed design uses more than one longitudinal coil to obtain a reasonably large stroke. A larger stroke reduces the force required and results in a higher linear velocity. Both effects allow a higher efficiency to be obtained. Because of the direct drive with few other losses, overall blood pump efficiency is calculated to be 20% which is as good as with present art high speed rotary motor actuators. The proposed design rivals the most efficient blood pumps to date requiring only 9 watts of power and weighs only 1.3 pounds.
The instant invention also uses coils on each side of the moving magnet unlike other voice coil designs. This reduces the side loads on the magnet and makes the use of a hydrodynamic linear oil bearing practical for supporting the moving magnet assembly.
All linear actuators need bearing support of some kind. Ball bearings are used as rollers in commercial linear motors but these pose severe space limitations if one wants to minimize the size of a blood pump. They also wear and are subject to ball indentation damage from shock loads. The proposed oil bearing is ideally suited to this application which requires the motor to run continuously. The bearing can also sustain very high shock loads without damage. This is due to the squeeze film effect in addition to the hydrodynamically generated oil film. Of equal importance, no additional moving parts are introduced by the bearing. This is very important because reliability has been a major development problem. The ball bearings used in high speed rotary motors fail prematurely, particularly when they are reversed in direction at high sped. They have to undergo billions of cycles and each cycle produces wear and fatigue.
Another advantage offered by the instant invention is one of small size and weight. The direct drive approach eliminates the need for motion conversion mechanisms which are generally larger and heavier than the high speed rotary motor used. They also reduce overall efficiency.
The highly effective forced conversion cooling of the motor using the available hydraulic fluid (oil), also distinguishes this design from voice coil designs of prior art that use ineffective air cooling (such as the loud speaker).
Having discussed the main disadvantages in the actuators used in blood pumps presently under development, such as the existence of parts that wear, low mechanical reliability, the use of many moving parts, their larger size and weight; the following objects of the instant invention can be stated.
A primary object of this invention is to eliminate all wear. This is accomplished by using a self acting linear hydrodynamic oil bearing to support a linear motor. Commercial linear motors sometimes use hydrostatic air bearings for accurate positioning requirements. These bearings are supplied with an externally supplied source of air not practical in this application. Another object of this invention is to maximize reliability by having only one moving part. This is accomplished by the synergistic use of a moving magnet linear motor with an attached hydraulic piston. The oil bearing is integral with this moving assembly. Another object is to provide a double acting hydrodynamic bearing that functions in two directions of motion. A back to back configuration accomplishes this goal where the center zone between respective halves of the bearing can communicate with ambient oil to isolate each half of the bearing. Another object of the invention is for the linear bearing to synergistically act as the hydraulic piston to pump hydraulic fluid alternately back and forth to left and right ventricles. This is accomplished by the bearing acting as both a clearance seal and a piston located between the two ventricles.
Yet another object of this invention is to provide a linear actuator that exhibits low side loads. This is accomplished by surrounding the moving magnet with drive coils at both its inner and outer diameters. The resulting large air gaps with the stator iron cause low radial instability forces because the magnet ring is kept far from the stator iron. Another object of this invention is to hydraulically couple the linear motor to a variety of blood pumping ventricles that expel blood using suitable one way valves. Another object of the instant invention is to provide very efficient liquid cooling for the linear motor. This is accomplished by submersing the entire motor in the same hydraulic fluid as used by the bearing and used to compress the ventricles. Provisions are made to cool the motor coils by direct contact with the oil and by proportioning the flow of oil over specified areas of the stator. The piston motion creates the driving force for the forces convection of oil flow. This heat is then transferred to the blood via oil convection over the ventricles. Another object of the instant invention is to produce a linear actuator that is efficient and low in weight. Some present day artificial hearts weigh upwards of two pounds where as the human heart typically weighs less than a pound. This is accomplished by utilizing, in the preferred embodiment, a double coil motor. Its stroke can be optimized and be more efficient than single coil designs used for small strokes. The forced liquid cooling of the stator by the piston also permits designing a smaller lighter weight motor.
Another object of the present invention is to provide for closed loop control of the actuator. This is accomplished by employing a position sensor to give piston position and velocity information. This sensor is also used to switch the motor coils for commutation. Yet another object of the invention is to provide a control to maintain stable location of the ventricles. The hydraulic fluid will preferentially tend to leak past the piston from left to right due to the higher pumping pressure at the left ventricle. Hence both ventricles will tend to drift toward the right side. A ventricle position sensor is used to correct the position of the ventricles.
These and other objects of the present invention are accomplished in accordance with a preferred embodiment. For illustrative purposes only, a linear motor is positioned between right and left blood pumping ventricles. A radially magnetized magnet ring is positioned inside the motor and is the only moving element of the actuator. Its radial flux passes into a surrounding stationary ferromagnetic return shell after first passing radially through coils positioned on the inside and outside of the magnet ring. The return shell or stator recirculates the flux back to the magnet. Sufficiently large air gaps separate the magnet from the coils so that no wear or contact occurs at these locations.
A piston or hyraulic pusher plate is attached to the magnet. The piston is as long as practicable to act as a support bearing. A set of double acting tapers or steps are located on its surface to generate the hydrodynamic oil film. This film separates the sliding surfaces and eliminates all wear for indefinitely long life. The bearing also serves as a piston clearance seal to alternately pump hydraulic fluid to each ventricle. The oil flow is forced to go over the motor surfaces thereby providing very efficient forced convection cooling. This allows the motor to be of minimum weight.
An annular groove located at one end of the bearing housing, is used to provide reverse leakage past the bearing when the port is intentionally overlapped by a hole in the piston. This port than functions like a valve. It allows one to compensate for long term preferential leakage past the bearing by permitting hydraulic fluid to leak back from right to left. A ventricle position sensor is used to determine when correction is needed.
The motor is cooled by oil, forced to flow by the motion of the piston, over the outside of its housing. Exit holes are provided in the housing for the oil. The movement of the magnet ring also pumps oil in and out of the confined space at the coils. Holes are provided for this flow to exit to the ventricles. A central hole in the stator also circulates oil driven by the piston. An optional nonmagnetic plug can limit or proportion this flow if desired. By proportioning the sizes of the oil exit holes the cooling flow can be optimally proportioned in areas of the motor to optimize heat extraction.
The piston bearing effectively separates the blood pump into left and right halves so that its reciprocating action alternately pressurizes the two ventricles. The ventricles may be of commonly accepted construction using seamless polyurethane compliant sacks. The inner surface of the sacks generally designated 36 is in contact with the oil while their outer surfaces 37 contact a supporting structure 27. One inlet and one outlet valve is used in each ventricle to achieve unidirectional pumping action. The proposed actuator concept can be applied to an endless variety of ventricle materials and shapes. A compliance chamber is not required for proper operation as the mechanism runs at constant internal volume.